1. Technical Field of the Invention
The present invention relates generally to semiconductor devices and more specifically to methods for forming a semiconductor substrate having electrically isolated surface regions.
2. Description of the Prior Art
Conventional bulk semiconductor devices are formed in semiconductor material by implanting wells of P-type or N-type material in a wafer of the other type of material. Gates and source-drain areas may then be manufactured on the wafer using well-known processes. The resulting devices are known as metal oxide semiconductor (MOS) field effect transistors (MOSFETs). Each device must be electrically isolated from others on the same circuit in order to avoid unwanted connections in the circuits. A relatively large amount of surface area is needed for the electrical isolation of the various MOSFETs. This large amount of surface area devoted to isolation is undesirable in view of the trend toward reduced size of integrated circuits and greater density of elements on integrated circuits. Prior methods of electrically isolating semiconductor integrated transistors from one another have included laterally isolating active regions using insulating material. Two methods of forming the insulating material are (1) selective oxidation of wafer semiconductor material surrounding the active regions by processes such as low temperature oxidation (LTO) or local oxidation of silicon (LOCOS) and (2) deposit of insulating material in trenches formed around the active regions. Such techniques are of limited applicability in that they form insulating material only in relatively shallow regions around the active regions.
Another method that has been utilized is silicon-on-insulator (SOI) isolation techniques. In SOI techniques active areas are formed on an insulating substrate or layer, thereby providing more complete insulation between adjacent active areas of an integrated circuit. However, SOI techniques suffer from problems such as dynamic floating body effects. Floating body effects occur when the body of a device is not connected to a fixed potential and therefore the device takes on a charge based on the history of the device. This can be particularly detrimental in devices such as dynamic random access memory (DRAM) devices where it is critical that a pass transistor stays in an "off" condition to prevent charge leakage from a storage capacitor.
Accordingly, from the above it is seen that there is a need for devices that provide good electrical isolation such as SOI devices without the undesirable drawback of floating body effects.